(1) Field of the Invention
This invention relates to an apparatus for controlling the flow of an electrically conducting medium along a surface and more particularly to an array of magnets and electrodes for actively controlling the naturally occurring microturbulence in the boundary layer adjacent the surface.
(2) Description of the Prior Art
It is well known in the art that control of turbulence and vorticity in the boundary layer flow around a surface moving through a medium, such as a marine hull moving through seawater, reduces both drag and radiated noise. Additionally, fluid dynamically loaded surfaces, such as diving planes and rudders, or thrusting surfaces, such as propellers or screws, can operate at higher performance coefficients when boundary layer control systems are used. Historically, there have been two approaches to reducing drag or turbulence: delaying the laminar to turbulent transition by maintaining laminar flow near the surface; and reducing turbulence intensity in a turbulent state. Various approaches to maintaining laminar flow have been tried including shaping of the surface, heating the surface and providing suction at the surface to prevent or delay the transition into turbulent flow. However, laminar flow controls normally require very stringent surface smoothness, i.e., to within several tens of microns. Further, laminar flow control is susceptible to ambient disturbances which can trigger the transition from laminar to turbulent flow. Also, such control methods may result in large volumetric space penalties. Turbulent drag reduction efforts include injection of a polymer or microbubbles into the boundary layer flow adjacent the surface, vortex generators and electromagnetic control in electrically conducting media. Polymer and microbubble injection require additional space and weight for carrying the injection media. Vortex generators require movable wetted areas. Electromagnetic control of turbulence through the application of Lorentz forces holds very high potential for turbulent drag reduction. However, in the current state of the art, as exemplified by Nosenchuck et al., U.S. Pat. Nos. 5,320,309 and 5,437,421, the application of forces is not optimized for the flow conditions over the surface. The spacing of the Lorentz force generators and the timing of the application of the Lorentz forces are not consistent with the microturbulent phenomenology. For the checkerboard pattern of Lorentz force generators in the '421 patent, recent experimental results indicate minimal drag reductions for the majority of test cases with a small number of tests having skin friction reductions of up to 8%.